1. Technical Field
The present invention relates to a hologram transfer foil that enables a volume hologram to be applied by thermo-compression onto any desired application member, and a volume hologram multilayer structure obtained by use of said transfer foil.
2. Related Art
A volume hologram provides a means capable of recording information in its thickness direction, and recording and reconstructing 3M images. Volume holograms are now fabricated by known fabrication processes. Typically, the volume holograms are used for prevention of illegal copying of ID cards, bank cards, etc., because they are fabricated by precise operations using the optical equipment and so are very difficult to fake. In addition, the volume holograms are expressed in interference colors of light and so have an outside appearance hardly achievable by other imaging means.
In a typical application taking full advantage of the aforesaid features of volume holograms, a hologram adhesive label, obtained in a film form with an adhesive attached thereto, is applied to a variety of associated members. For instance, JP-A-63-284586 discloses a transfer type hologram of a multilayer structure comprising a substrate, a volume hologram layer and a heat seal layer in this order. In view of illegal copying using a relief hologram, for instance, Japanese Patent No. 2,934,281 shows a transfer type hologram of a multilayer structure comprising a substrate, a relief hologram layer and a heat seal layer in this order.
In the former, however, the volume hologram layer is basically made up of a photocured resin layer whereas the associated member is formed of high-quality paper or the like. Such a material difference often makes it difficult to select a heat sealing agent showing satisfactory adhesion with respect to both materials, and so this label is applied to only a limited number of application members although depending on the heat sealing agent used. For this reason, the hologram layer peels off at an interface between the volume hologram layer and the heat seal layer or the heat seal layer and the application member, resulting in a high risk of illegal copying. A problem with the latter is that characters, images or the like on the application members cannot be seen through.
In view of prevention of illegal copying, the volume hologram layer must have good adhesion to the application member, and one approach to improvements in the adhesion between a volume hologram layer and an application member is typically set forth in JP-A-2002-358018 showing that the volume hologram layer and the application member are bonded together via a plurality of heat seal layers, and exemplifying a polyvinyl chloride or polyester resin sheet as the application member. However, the polyvinyl chloride sheet offers an environmental pollution problem arising from the generation of dioxin, etc. upon incineration and disposal. In recent years, therefore, the use of polyester resin sheets as application members has grown. Still, polyester resin sheets are lesser adhesive than polyvinyl chloride cards, and there is insufficient adhesion to the volume hologram layer, especially a volume hologram layer comprising a photocation type monomer and a photoradical type monomer, often offering a problem in conjunction with illegal copying. Multiple heat seal layers are unfavorable for fabrication, and so it is desired that the volume hologram layer and the application member be bonded together via a single-layer heat seal layer. What is especially in need is the development of a heat seal layer fit for the use of a polyester resin sheet as an application member.
The present invention has for its object the provision of a counterfeit resistant volume hologram transfer foil which enables a volume hologram layer to be well bonded by thermo-compression to any desired application member, especially a polyester resin card, and which, even when a volume hologram layer is formed of a volume hologram photosensitive material comprising a cation-polymerizable compound and a radical-polymerizable compound, is well bonded by thermo-compression to an application member as well as a volume hologram multilayer structure formed by use of such a transfer foil.